1. Field of the Invention
The present invention relates to a halographic head-up display apparatus, and more specifically to a holographic display apparatus provided with means for eliminating blur (unclearness is displayed images) due to chromatic aberration caused through the display apparatus.
2. Description of the Prior Art
An example of holographic head-up display apparatus is disclosed in U.S. Pat. No. 4218111. In this patent, a series of relay lens and a narrow-band high luminance cathode ray tube (CRT) are incorporated in the apparatus in order to eliminate the influence of chromatic aberration upon displayed images which are produced by projecting CRT display images on a reflective hologram diffraction grating (plate). Therefore, this display apparatus involves some disadvantages such that the optical system is complicated and therefore the light efficiency is low or the display apparatus is heavy and large in size.
To overcome the above-mentioned drawbacks, U.S. Pat. No. 4613200 (HEAD-UP DISPLAY SYSTEM WITH HOLOGRAPHIC DISPERSION CORRECTING) discloses another display apparatus such that a pair of separate reflective hologram diffraction gratings manufactured under the same manufacturing conditions are arranged in parallel to each other in order to compensate a display image for chromatic aberration. In this apparatus, however, there exist other problems such that the two hologram diffraction gratings are required to be arranged under severe restriction or it is difficult to perfectly coincide the characteristics of two hologram diffraction gratings with each other.
The above-mentioned two U.S. Patent Documents are incorporated herein by reference.
Further, in this specification, a holographic head-up display implies an image obtained by a combination of an outside direct view as for example that of a car driver with superimposed light from another source (instrument images) by use of diffraction optics as a combining and collimating element.